Microporous materials, including zeolites and silicoaluminophosphates, are widely used in the petroleum industry as absorbents, catalysts and catalyst supports. Their crystalline structures consist of three-dimensional frameworks containing uniform pore openings, channels and internal cages of dimensions (<20 Å) similar to most hydrocarbons. The composition of the frameworks can be such that they are anionic, which requires the presence of non-framework cations to balance the negative charge. These non-framework cations, such as alkali or alkaline earth metal cations, are exchangeable, either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. If these non-framework cations are converted to the proton form by, for example, acid treatments or exchange with ammonium cations followed by calcination to remove the ammonia, it imparts the material with Bronsted acid sites having catalytic activity. The combination of acidity and restricted pore openings gives these materials catalytic properties unavailable with other materials due to their ability to exclude or restrict some of the products, reactants, and/or transition states in many reactions.
Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are sometimes referred to as “molecular sieves” because interconnecting channel systems created by pores of uniform pore size allow a zeolite to selectively absorb molecules of certain dimensions and shapes
By way of background, one authority has described the zeolites structurally, as “framework” aluminosilicates which are based on an infinitely extending three-dimensional network of AlO4 and SiO4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula:M2/nO·Al2O3·xSiO2·yH2O
In this empirical formula, x is equal to or greater than 2, since AlO4tetrahedra are joined only to SiO4 tetrahedra, and n is the valence of the cation as designated in M. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite beta (U.S. Pat. No. 3,308,069 and RE 28341); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The silicon/aluminum atomic ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with silicon/aluminum atomic ratios of from 1 to 1.5, while that ratio in zeolite Y is from 1.5 to 3. In some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. U.S. Pat. No. 3,941,871, reissued as RE. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 zeolites.
Moreover, the silicon/aluminum atomic ratio of the “as-synthesized” zeolite can be altered, specifically increased, by decreasing the tetrahedral aluminum thereof. Decrease in the tetrahedral aluminum may be affected by synthetic methods developed to deplete the tetrahedral aluminum of a zeolite. In addition, the silicon:aluminum atomic ratio of a zeolite may be increased, that is there may be a loss of tetrahedral aluminum, as a result of process conditions to which the zeolite may be subjected during use. Process conditions which will effect depletion of tetrahedral aluminum include high temperature calcination and steaming. This loss of aluminum does not affect the crystallinity of zeolites, such as ZSM-5.
Those zeolites of practical significance today are not only characterized by uniform pore sizes, but also by channel systems created by those pores. To maintain activity of a zeolite, the crystallographic structure of the zeolite after chemical treatment must remain intact.
The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal, an alkaline earth metal or an organic cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The cavities and pores are occupied by molecules of water prior to dehydration and/or possibly by organic species from the synthesis mixture in the as-synthesized materials.
Numerous methods have been developed to increase the activity of catalysts. Zeolite acid activity can be increased by various means such as mild steaming, hydrothermal treatment in the presence of aluminum, and vapor phase treatment with aluminum chloride. Various chemical treatments of zeolites have been proposed to modify their chemical properties and increase catalyst activity. U.S. Pat. No. 4,444,900 teaches a technique to wash the catalyst with dilute hydrofluoric acid to de-te the catalyst in order to increase the sites for ion exchange. U.S. Pat. No. 6,124,228 teaches a standard method of increasing catalyst activity by performing an ion exchange with an ammonium salt followed by calcination. The '228 patent also teaches using potassium nitrate as opposed to an ammonium salt. U.S. Pat. No. 6,207,042, which also mentions that potassium nitrate may be used as an ion-exchange step, teaches that this will actually reduce the acidity (and thus activity) of the catalyst. The activated or acidified form of the zeolite is often referred to as the H-zeolite, H-form zeolite or the proton form of the zeolite.
U.S. Pat. No. 4,265,788 teaches simultaneously using both an ammonium nitrate and potassium nitrate ion-exchange procedure. However, the '788 patent teaches nothing concerning increasing the activity of a catalyst; rather it teaches a procedure to leave the potassium ion in the zeolite's channels allowing for the selective separation of para-xylene from other C8 aromatic hydrocarbons. The inventors are not aware of any prior art that teaches an ion-exchange first with a potassium ion, followed by an ion-exchange with an ammonium ion to increase catalytic activity of a zeolite.
The sodium content of a catalyst also has been shown to be important to the catalyst's activity. U.S. Pat. No. 6,207,042 teaches that activity and anti-fouling improvements can be achieved by removing as little as an additional 0.1 wt % sodium from the catalyst matrix. The manufacture of commercial catalysts often leaves substantial quantities of sodium in the crystal to offset the aluminum charge deficiency compared to the silica. The sodium is commercially exchanged out of the crystal with ammonium nitrate producing the well-known H-form of the zeolite after calcination. Depending on the particular zeolite, however, fairly large quantities of sodium remain in the crystal, negatively affecting the activity of the H-form.
While previous patents have taught ion exchange methods using ammonium salts or potassium salts, the inventors have unexpectedly found that a serial ion exchange, first by one or more exchanges with a potassium salt, followed by one or more exchanges with an ammonium salt dramatically reduces sodium content and increases catalyst acidity and activity.